As Reed Richards for trapping him as a 'monster.' Unfortunately for the ever-lovin' blue-eyed Thing, he's the one who broke his powers - and then blamed his best friend.
The Thing #22 by John Byrne, Ron Wilson, Joe Sinnott, John Morelli, and Bob Sharen showcases the ending of a 12-issue saga in the life of Ben Grimm as he survives alone on Battleworld following the end of the original Secret Wars. Ben decides to stay behind on Battleworld after discovering he can change forms between the Thing and his human state on the patchwork planet. At the end of The Thing #22, the Thing's Battleworld lover Tarianna kills a mental construct of Ben Grimm's human form, consequently blocking Ben from ever returning to a human state. It is here that Ben discovers a shocking truth: ever since he got his powers, he always had the ability to change back into his human form, whether on Battleworld or off. When his human form dies on Battleworld, however, this power dies with it.
The Thing Destroyed His Own Chance to Change Back
Little did Ben know, this revelation about his powers wasn't a surprise to everyone. Before leaving his best friend behind on Battleworld, Reed suspected that Ben already held the power to change back at will. Reed noticed multiple times (showcased in Byrne's run on Fantastic Four) that when Ben was presented with failed opportunities to return to his normal body, he would feel relief in remaining as he was. This was due to Ben's fear that his girlfriend Alicia (now his wife, following their recent marriage in ongoing canon) only loved him as the Thing and not as a normal human. When Reed saw Ben able to change forms on Battleworld, he deduced that this was actually a power his best friend had always had; it was his fear of Alicia's judgment that kept him from unlocking this ability within him.
The Thing Wasn't Originally Trapped in One Form
The knowledge that Reed hid this power from him the whole time caused a nearly irreparable rift in the Thing's relationship with Mr. Fantastic and the Fantastic Four as a whole. Since Reed left Ben on Battleworld believing that the planet held the key to his transformation, the Thing, known for having a temper, felt that his former best friend was complicit (and even the cause of) the death of his hidden power to change form. The Thing argued that had Reed just told him that his insecurity with Alicia was holding back his transformation, he may never have stayed behind on Battleworld in the first place, and Tarianna would have never killed his human form. This - coupled with the fact that Johnny Storm had seemingly begun dating Alicia in his absence - was enough to cause Ben to quit the Fantastic Four altogether, allowing the Sensational She-Hulk to keep his spot on the team.
What Ben didn't it to Reed is that he may have had a hand in this power's destruction himself. When Reed left the Thing behind on Battleworld, he warned his friend that everything that happened on the strange planet would in some way reflect Ben's own desires. When Tarianna (herself a creation of Ben's subconscious wants) "killed" Ben Grimm, the Thing realized that he subconsciously wanted to remain more than human. Reed Richards is constantly blamed for trapping Ben Grimm as the Thing, but the truth is that for much of Fantastic Four history, Ben's own neuroses were the only thing stopping him turning human, and his own adventures (which Reed warned him about) destroyed his chance to access this power.
